CUDA Graph Algorithm Optimization Guide

// Vertex-parallel: one thread per vertex
__global__ void bfs_vertex_parallel(int* row_ptr, int* col_idx,
                                    int* distances, int* frontier,
                                    int num_vertices, int level) {
    int v = blockIdx.x * blockDim.x + threadIdx.x;
    if (v >= num_vertices) return;

    if (distances[v] == level) {
        // Process all neighbors
        for (int e = row_ptr[v]; e < row_ptr[v + 1]; e++) {
            int neighbor = col_idx[e];
            if (distances[neighbor] == -1) {
                distances[neighbor] = level + 1;
                frontier[neighbor] = 1;
            }
        }
    }
}

// Edge-parallel: threads assigned to edges
// Step 1: Expand frontier vertices to edge list
// Step 2: Process edges in parallel

// CSR to edge list expansion
__global__ void expand_frontier(int* row_ptr, int* frontier_vertices,
                                int* edge_src, int num_frontier) {
    // Each frontier vertex's edges expanded
}

// Edge-parallel processing
__global__ void bfs_edge_parallel(int* edge_src, int* edge_dst,
                                  int* distances, int* new_frontier,
                                  int num_edges, int level) {
    int e = blockIdx.x * blockDim.x + threadIdx.x;
    if (e >= num_edges) return;

    int src = edge_src[e];
    int dst = edge_dst[e];

    if (distances[src] == level) {
        if (atomicCAS(&distances[dst], -1, level + 1) == -1) {
            // First to visit this vertex
            new_frontier[atomicAdd(&frontier_count, 1)] = dst;
        }
    }
}

// For high-degree vertices, use work group assignment
__global__ void bfs_load_balanced(int* row_ptr, int* col_idx,
                                  int* distances, int level,
                                  int* vertex_to_warp, int* warp_offsets) {
    // High-degree vertices split across warps
    // Each warp processes subset of edges
    int warp_id = /* warp assignment */;
    int vertex = vertex_to_warp[warp_id];
    int edge_start = warp_offsets[warp_id];
    int edge_end = warp_offsets[warp_id + 1];

    for (int e = edge_start + threadIdx.x % 32; e < edge_end; e += 32) {
        int neighbor = col_idx[e];
        // Process neighbor...
    }
}